The invention relates to a method of manufacturing electronic devices, in particular, but not exclusively, semiconductor devices, in which method wafers, which are provided at a surface with a material to be removed, are subjected, while being divided into successive batches, to a wet treatment in a bath containing a solution of an active component in a solvent, during which wet treatment the successive batches of wafers are immersed in the solution at first time intervals, which first time intervals each consist of a processing period during which the material is removed from the surface of the wafers, thereby forming ionic components, and a waiting period following the removal of the wafers from the bath at the end of the processing period, during which wet treatment of the successive batches the conductivity of the solution is monitored, which monitored conductivity is brought to approximately a desired conductivity at second time intervals during the wet treatment of the successive batches by adding at least one of the active component and the solvent to the solution inside the bath.
An important wet treatment in IC manufacturing technology involves wet etching of silicon oxide, which is frequently carried out in a bath containing a hydrofluoric acid solution (HF/H2O). Besides etching of silicon oxide, hydrofluoric acid solutions may also be used for etching of, for example, silicon nitride and silicon oxynitride. Other important wet treatments make use of, for example, the widely known RCA solutions, i.e. the ammonia hydrogen peroxide solution (NH4OH/H2O2/H2O), which is effectively used for particle removal with (re-)growth of clean chemical oxide on silicon surfaces and is known as standard clean 1 (SC1), and the hydrochloric acid hydrogen peroxide solution (HCl/H2O2/H2O), which is effectively used for metal removal and is known as standard clean 2 (SC2).
Consumption of components of the solution as a result of the removal of material from the surface of the wafers, evaporation of volatile components from the solution, and drag-out of components from the solution as a result of the removal of wafers from the bath are important factors influencing the chemical composition of the solution in the course of time. As the rate of etching or removal depends inter alia on the chemical composition of the solution, it is important to monitor and control the chemical composition of the solution, so as to achieve a better process control. An important category of techniques applied for monitoring and controlling the chemical composition of solutions are based on measurement of the solution conductivity in the course of time.
A method of the kind mentioned in the opening paragraph is known from an article entitled “In-situ chemical concentration control for wafer wet cleaning”, written by Ismail Kashkoush et al. and published in Mat. Res. Soc. Symp. Proc., Vol. 477 (1997), pp. 311-316. In this method, the monitored conductivity of a hydrofluoric acid solution (HF/H2O) is brought to approximately the desired conductivity at the second time intervals by adding hydrofluoric acid and/or deionized water. The monitored conductivity of an ammonia hydrogen peroxide solution (NH4OH/H2O2/H2O) is brought to approximately the desired conductivity at the second time intervals by adding ammonia and/or deionized water, whereas hydrogen chloride and/or deionized water are added in the case of a hydrochloric acid hydrogen peroxide solution (HCl/H2O2/H2O). As the fluctuations in the concentration of hydrogen peroxide in both RCA solutions, and the effect of hydrogen peroxide on the conductivity of these solutions are minimal, hydrogen peroxide is added independently according to defined time intervals.
It has been experimentally observed that, although the conductivity of the solution inside the bath is accurately controlled by bringing the monitored conductivity to approximately the desired conductivity at the second time intervals, a decrease in removal or etch rate of the material takes place from batch to batch during the wet treatment of the successive batches of wafers. In this way, a deterioration of the process stability of the wet treatment of the successive batches takes place, and the lifetime of the bath containing the solution is shortened.